General purpose optical encoders are common for position and motion sensing in a variety of systems that require accurate positioning of equipment such as telescopes, milling machines, lathes, etc. Common encoders include rotary and linear types in which a rotary encoder uses a code wheel and a linear encoder uses a linear scale. Both types of encoders are provided with a light source, photodetectors and a signal processor.
A basic encoder operates by emitting a light beam from a source through a collimating lens to a linear or rotary scale which typically includes a grating of opaque and transparent sections. The collimated light beam passes through the transparent sections to appropriately positioned photodetectors so that the photodetectors receive a high intensity of light during transmission and no light when the opaque sections block the collimated light. A conventional encoding system is intensity sensitive, so that the detection of diffracted light by the photodetectors introduces detection error. Although a fine grating is desirable to improve detection accuracy, a relatively coarse grating is necessary to minimize diffraction of light and the resultant detection error.
U.S. Pat. No. 5,000,572 issued 19 Mar. 1991 to Nose et al. discloses a distance measuring system for measuring a distance between two objects by detecting the phase of diffracted light. Specifically, the distance measuring system includes: a first diffraction grating provided on one of the two objects and disposed along the relatively moving direction of the two objects; and a measuring portion provided on the other object. The measuring portion includes a second diffraction grating, a light source and a photodetecting system, wherein the light source provides lights which are projected upon two points on the second diffraction grating so that they emanate from the two points in the form of diffraction lights having different orders. The diffraction lights are directed to the same point on the first diffraction grating and are diffracted again by the first diffraction grating so that they are emitted in the same direction, and the photodetecting system is operable to detect a change in the light intensity caused by the interference of the two lights emanating from the first diffraction grating. The distance measuring system further includes a detecting system for detecting the relative moving distance of the two objects on the basis of the detection by the photodetecting system. As described above, the Nose distance measuring system measures the phase of diffracted light using two diffractive gratings with the accompanying optics and electrical circuitry.
U.S. patent application Ser. No. 08/201,256, filed on 24 Feb. 1994 as a continuation-in-part of abandoned U.S. application No. 07/776,565 filed 15 Oct. 1991 (both by Paul F. Sullivan), discloses a different light beam position detection and control apparatus which employs diffraction patterns. More specifically, a system is disclosed for accurate one dimensional positioning of an object using three repeating diffractive patterns or two-dimensional positioning using six superimposed patterns. Although the disclosed encoding system exhibits excellent, improved performance over the prior art, the output of the system does not conform to the industry standard for encoder outputs having two signals which are 90 degrees out of phase with one another. Dual encoder outputs are typically in quadrature to match the input requirements of readouts and controllers of motor drivers. One standard incremental optical linear encoder is model SRL manufactured by DRC which specifies symmetry at 180.degree..+-.10.degree., quadrature at 90.degree..+-.45.degree. and maximum operating speed at 50 inches per second.
Consequently, it is a primary object of the invention to provide a general purpose encoder for position detection and control which offers improved resolution performance over the above and other prior teachings by accurately detecting diffracted light beams and generating two outputs in quadrature.
It is another object of the invention to provide a general purpose quadrature diffractive encoder with improved electrical and mechanical characteristics.
The above and other objects will, in part, appear hereinafter and, in part, be obvious when the following detailed description is read in conjunction with the drawings.